The recent explosion of information concerning the molecular mechanisms underlying circadian rhythmicity in organisms as diverse as bacteria and mammals has left unanswered critical questions about the organization of circadian systems in multi-cellular organisms. At the same time, it has provided important new tools that can be used to answer these questions “A. J. Millar, S. R. Short, N. H. Chua, S. A. Kay, Plant Cell, 1992, 4, 1075; T. Kondo et al., Proc. Natl. Acad. Sci. U.S.A., 1993, 90, 5672; J. D. Plauts, M. Kaneko, J. C. Hall, S. Kay, Science, 1997, 278, 1632”.
Although the suprachiasmatic nucleus (SCN) is the dominant circadian pacemaker in mammals, a surprising number of peripheral tissues appear to contain the molecular machinery necessary for circadian oscillation “U. Albrecht, Z. S. Sun, G. Eichele, C. C. Lee, Cell, 1997, 91, 1055; D. P. King, et al., Cell, 1997, 89, 641; L. P. Shearman, M. J. Zylka, D. R. Weaver, L. F. Kolakowski Jr., S. M. Reppert, Neuron, 1997, 19, 1261; H. Tei, et al., Nature, 1997, 389, 512; N. Koike, et. al., FEBS letters, 1998, 441, 427; Y. Miyamoto and A. Sancar, Proc. Natl. Acad. Sci. USA., 1998, 95, 6097; M. J. Zylka, L. P. Shearman, D. R. Weaver, S. M. Reppert, Neuron, 1998, 20, 1103” and, in a few cases, these vertebrate tissues have been shown to express circadian oscillations in the absence of the SCN “R. V. Andrews, Gegenbauers Morph. Jahrb. Leipzing, 1971, 117, 89; G. Tosini and M. Menaker, Science, 1996, 272, 419; A. Balsalobre, F. Damiola, U. Schibler, Cell, 1998, 93, 929; D. Whitmore, N. S. Foulkes, U. Strahle, P. Sassone-Corsi, Nat Neurosci, 1998, 1, 701”. However, detailed relationships among the oscillations in multi-tissue systems remain unknown.